Electrolyte conductivity is a measure of a solution's ability to carry electric current. In an otherwise relatively nonconducting solution, the measure of electrolyte conductivity can provide a nonspecific though quantitative indication of the total level of ionized impurities present. Owing to the simplicity and relatively low cost of commercially available conductivity cells and meters, this technique has found widespread acceptance in many modern manufacturing processes. In the production of ultrapure water, for example, where large quantities of high purified water must be monitored to maintain the purity, conductivity measurements provide the principal measure of purity.
Specific conductance is defined as the conductance in mhos of one centimeter cube of the liquid at a specified temperature, commonly 25.degree. C. Inversely, specific resistance is similarly defined as resistance in ohms of a one centimeter cube of the liquid at a specified temperature. The units of specific conductance and specific resistance are mhos/cm and ohm-cm respectively. Distilled water typically has a specific conductivity at room temperature of just less than one micromho/cm. Absolutely pure water has a conductivity of 0.055 micromho/cm. at 25.degree. C.
Techniques for the measurement of electrolyte conductivity are well known in the prior art. Modern direct contact liquid conductivity sensing instruments generally comprise two electrodes positioned within the solution whose electrical resistance or conductance is to be measured. The electrodes are used to apply a constant voltage across a known volume of liquid, and the resultant current therethrough is measured. If a direct current voltage is applied across the cell electrodes, ion migration occurs, leading to inaccuracies due to the resultant changes in the composition of the solution adjacent to the electrodes. This effect is termed polarization. Therefore, alternating current is almost universally employed.
In addition to polarization effects induced by the measuring circuit, it has been found by the present inventor, and the literature shows, that polarization can occur due to both kinetic and chemical effects within the solution itself. If active chemical reactions are occurring in the sample solution, this effect can be quite pronounced. For example, see the copening application of Frederick K. Blades, et al, Ser No. 635,551, filed Aug. 2, 1984, incorporated by reference herein, in which an instrument is described having conductivity measuring electrodes exposed directly to short-wave ultraviolet radiation. It has been found that under these circumstances, the electrochemical activity at the surface of the electrodes, stimulated by the heavily oxidative environment caused by the presence of ultraviolet radiation, produces a net potential across the electrodes on the order of several hundred millivolts. Accurate compensation techniques must be employed if the solution conductivity in the presence of such an induced potential is to be accurately measured.
Accordingly, it is an object of the present invention to provide a circuit by which the effects of an induced, ordinarily slowly varying potential are accurately compensated for.
As mentioned, the instrument described in the copending application referred to above comprises a low pressure discharge ultraviolet light source situated in proximity to the conductivity measuring electrodes. The high voltage excitation required to operate such a lamp tends to induce noise in the measuring circuits, and it was found desirable to add a filter to attenuate the lamp noise in series with the conductivity measuring circuit. However, the effect of adding the filter is to introduce a constant resistance in series with the solution resistance being measured. A constant resistance in series with a varying solution resistance produces an undesirable non-linearity in the output signal.
Accordingly, it is a further object of the present invention to provide a circuit by which the non-linearity produced by a constant resistance in series with the cell resistance is accurately compensated for.